One of my favourite opening scenes of any film is that of Fellini's La Dolce Vita. As decadent young Romans disport themselves in what Fellini wants us to see as a city in the process of losing its soul, a helicopter lifts a giant statue of Christ above the rooftops and away into the hills. From then on, Fellini's celluloid Rome is a city in which natural goodness and natural order is drained away, so that all that is left by the time the credits roll is the flakey husk of human and man-made beauty.

It is at this time of year, when even British cities begin to feel a little like Rome and office workers flock, red-skinned, into parks for sandwich lunches and suntans, that most of us begin to ache for a little Nature in our lives. Although many readers of this paper will have access to a private garden, tens of thousands of city dwellers lack even the smallest balcony. They rely on public parks, which vary from the sublime - Sefton, Regent's - to the squalid and sad - all those tiny former or would-be parks in and around housing estates and those, like, Lincoln's Inn Fields in the heart of London, that have been allowed to fall into decline.

We are meant to be good at green space and gardening in Britain and yet we display precious little imagination in the way we thread greenery through our city centres. Parks have existed for generations, and should, of course, be nurtured and tended with all the care we can give them. They are the lungs of our cities, and the best - such as the chain that connects Paddington and Bayswater with St James's and Westminster - are admired by visitors from around the world.

As far as the British are concerned, though, greenery, save for the odd clematis or honeysuckle, stops at ground level. Yet increasingly, largely because of the density and height of modern buildings in city centres, roofs have become an ideal setting for gardens in the sky. Imagine if we could develop a gentle but effective strategy for greening this urban roofscape. This would be relatively cheap, great fun, involve the imagination and green fingers of thousands of people not normally called on to design and create parts of the urban landscape. It would shape places that we could all enjoy and encourage more wildlife. The roofs of offices, warehouse and blocks of flats could be transformed very quickly, so that by the turn of the century, our millennial roofscape could be the envy of the world.

What's more, there is no reason why we cannot create extensive urban parks high above our streets, by linking buildings with bridges and boardwalks. This would imply a security risk in certain cases, but, again, most of us could only benefit from being able to walk across sections of a city centre from garden to garden across what would be, effectively, an elevated park. Such parks would open up pedestrian routes across city centres. They would save many journeys by car, bus and taxi, reducing pollution and making trips across town a pleasure rather than a penance. The technology to make flat roofs watertight has been with us for some while now, so there is little need to fear that gardens will be a threat to the structure of the building below. Where there is a risk, a garden could be designed along formal Japanese lines or in some other abstract form.

If this modest millennial proposal sounds pie in the sky, precedent proves otherwise. The Hanging Gardens at Semiramis, Babylon (in what is now Iraq) dated from the seventh century BC and were one of the seven wonders of the ancient world. Closer to home are the superb gardens created along the former suburban railway line that ran south-east out of Gare d'Austerlitz, Paris over a viaduct of brick arches several miles long. Last year, the arches, which begin where the ugly new Bastille Opera House ends, were reclaimed for shops, cafes and studios for artists, craftworkers and architects. These have been an instant success. But far more successful is the rooftop walk above them that stretches for over two miles - two miles of gardens and parks, with access to cafes and overlooked by flats old and new.

Many British cities were carved up ruthlessly by the arrival of the railway in the mid- to late 19th century and by the urban freeway a century later. Roads and railways have created space both alongside them and beneath them that is usually dead - redundant, unattractive and even frightening. Such space could easily be turned into urban parkland or green routes for pedestrians and cyclists through city centres.

The main difficulty in making such small-scale, gentle and potentially immensely popular projects happen in Britain is one of ownership of land and property. Where one landowner - whether Duke or elected local authority - has a grasp of an area, it is possible to carry out fairly radical and brave schemes. Where ownership is split amongst a vast number of landlords, many of them pension funds and property developers, it is difficult to push through public strategies even if everyone might gain from them and like them.

London, for example, would benefit enormously from having a strategic planning authority (small, energetic, creative and accountable) that took an overview of ways in which to further green the capital, to create green walks, roof gardens and to plant public and semi-public spaces, from the undersides of urban motorways to waste-tips. We have such gardening talent here, it seems a shame not to use it further to make our cities more delightful places to be in the summer.

For dedicated urbanists who like the hard-edged texture of city centres and who may balk at the idea of honeysuckle and wisteria tumbling down every modern building, there is no reason why urban gardens - on roofs and elsewhere - should not be made almost entirely of solid materials. Again, in recent years, Paris has paved the way. In the Parc de la Villette, for example, visitors descend tough-looking concrete stairs to walk through a musical bamboo garden. A what? A sunken garden composed of nothing but giraffe-high bamboo in which the sound of electronically-activated gongs and temple-chimes is mixed with the chirrups and warbling of baguette- fuelled sparrows. On paper it sounds horrid; in reality, it is an urban delight.

Unlike giant festivals and vast monuments, a thread of urban gardens weaving up and down Britain would be a present we could afford to give ourselves for the Millennium. Nature will do much of the work for us and bring heart and soul into breathless and god-forsaken city sites.